On-axis scanning electron beam computed tomography ("CT") systems are known in the art, and are described generally in U.S. Pat. No. 4,352,021 to Boyd, et al.), issued Sep. 28, 1982. The theory and implementation of devices to help control the electron beam in such systems is described in detail in several U.S. patents to Rand, et al., including U.S. Pat. Nos. 4,521,900 issued Jun. 4, 1985; 4,521,901 issued Jun. 4, 1985; 4,625,150 issued Nov. 25, 1986; 4,631,741 issued Dec. 23, 1986, and 5,193,105 issued Mar. 9, 1993. Applicants refer to and incorporate herein by reference each above listed patent.
As described in U.S. Pat. No. 4,521,900 to Rand, et al., in scanning electron beam CT systems, an electron beam is produced by an electron gun at the upstream end of an evacuated generally elongated and conical shaped housing chamber. A large electron gun potential (e.g., 130 kV) accelerates the electron beam downstream along a first straight line path. Further downstream, the beam optical system, which includes focus and deflection coils, deflects the beam into a scanning path.
The deflected and focussed beam exits the beam optical system and scans an arc-shaped target that produces X-rays when impinged by the beam. The X-rays penetrate an object (e.g., a person) and are detected by an arc-shaped array of detectors. The target and array of detectors are mounted in a stationary gantry, and are concentric about the scanner axis (or "axis of symmetry"), a precisely defined axis that is perpendicular to the planes of the target and detector array. The detected data are computer processed to produce a CT reconstructed image of a portion (or slice) of the object.
Briefly, in the upstream chamber region between the electron gun and the focus and deflection coils a diverging electron beam is desired, but in the region downstream from the focus and deflection coils, a converging electron beam is desired.
In the upstream region, the electrons' space-charge advantageously causes the electron beam to diverge or expand. Expansion here is beneficial because the beam width at the target varies approximately inversely with the beam diameter at the focus and deflection coils. Unless removed from this region, positive ions can neutralize the space charge and prevent beam divergence, destabilizing or even collapsing the beam. Positive ions, which are produced from the interaction of the electron beam with gases remaining in the chamber after evacuation, are undesired in the upstream chamber region. An ion clearing electrode ("ICE"), coupled to perhaps a 1 kV potential, is mounted in the upstream chamber region to remove positive ions. The electron beam passes axially through the ICE, which creates a relatively large transverse electric field that sweeps away the slow moving positive ions, without disturbing the considerably faster moving electrons.
In the downstream region, positive ion neutralization is beneficial, since a converging, self-focussing, electron beam is desired. Elements in the beam optical system then fine tune the converged beam to produce a small electron beam spot and consequently a sharp X-ray image.
In summary, ideally, the electron gun and beam optics system are perfectly cylindrically symmetric, producing a perfectly homogenous electron beam having uniform electron distribution. Such an ideal beam would act as its own perfect lens: self-diverging in the upstream chamber region and self-converging in the downstream chamber region, to focus sharply on the target. In practice, if the electron gun is not ideal, beam uniformity should be corrected by the beam optical system.
In the prior art, the source of the electron beam (e.g., the electron gun, drift tube and beam optics) was coaxial with the scanner axis, forming an "on-axis" system. The configuration advantageously provided a substantially constant distance between the electron beam optic system and the arc-shaped target, facilitating maintenance of a sharply focused, elliptical shaped beam spot at all points along the scanned target.
Unfortunately, the on-axis configuration prevented the X-ray subject from passing completely through the gantry because the electron gun end of the conical chamber would be struck by the subject couch. By contrast, mechanical CT X-ray systems such as described in U.S., Pat. No. 4,630,202 to Mori permit the subject to move completely through a rotating gantry, but cannot provide sub-second single scans that eliminate motion artifacts, including heart motion, as can scanning electron beam CT systems.
What is needed is a scanning electron beam CT system that is off-axis, to allow an X-ray subject to pass completely through the gantry.
The present invention discloses such a system.